Semi-automatic and automatic firearms typically store cartridges in one of three ways: box magazines, drum magazines or belts.
Typical magazines are powered by a single spring whose function it is to move the cartridge upward toward the action as it cycles. This requires a spring which balances the speed necessary to move the entire stack of cartridges upward several times a second against the friction between the action and the top cartridge, which can tend to cause a stoppage. An overly powerful spring will create excessive friction, while a weak spring will fail to force a cartridge upward fast enough, creating stoppage. This balancing act must be successful for both a full magazine and a nearly empty one, as well as intermediate states, taking into account the changes in spring compression and total cartridge mass which occur as the gun is fired.
If the pressure exerted by the magazine on the top cartridge could be made more consistent regardless of the pressure exerted by the magazine's primary spring, then the balance would be much easier to strike, and more consistent and reliable feeding performance would be possible.